thank you for your help! why i thought of it for such a long time but couldn't see it. thanks again.
EDIT: ah.. TKhunny already gave the hint to me but i was in a hurry and didn't see it. how stupid i am....

thank you for your help! why i thought of it for such a long time but couldn't see it. thanks again.
EDIT: ah.. TKhunny already gave the hint to me but i was in a hurry and didn't see it. how stupid i am....

Not stupid...you've just not had enough experience at it. This is how you get experience.